5
6 of them.
There are 15180 combinations.
there are 10 possibilities for the first spot, 9 for the second, 8 for the third 10x9x8=720 combinations
There are 5,461,512 such combinations.
6 ways: 931,913,139,193,391,319
There are 1,120,529,256 combinations.
Only three: 12, 13 and 23. Remember that the combinations 12 and 21 are the same.
There are 9C3 = 84 combinations.
It depends on how many digit you are choosing from.
5
6 of them.
9
Only one.
How many four digit combinations can be made from the number nine? Example, 1+1+2+5=9.
120 combinations using each digit once per combination. There are 625 combinations if you can repeat the digits.
Six combinations if you use each digit only once per combination: 157 175 517 571 715 and 751.